1. Field of the Invention
The invention relates generally to heat exchange devices and more particularly to devices for cooling a habitable space using a combination of cooling principles. The invention further relates to methods of controlling such devices to operate together to achieve a desired effect.
2. Description of the Related Art
Heat exchangers of some form or another are to be found in almost every process. In general, processes produce heat and that heat must be removed in order for equilibrium to be maintained. In many processes, heat will be transferred away passively by a number of mechanisms, including conduction, convection, evaporation and radiation. In other situations, such natural processes are insufficient and specific cooling processes must be implemented to remove the undesired heat.
A widely used and well understood process for cooling is the refrigeration cycle in which a fluid is pumped between a heat source and a heat sink. At the heat source, the fluid in liquid form adsorbs heat and is caused to evaporate. By compressing the fluid between the heat source and the heat sink the temperature of the boiling point of the fluid is adjusted. The gaseous fluid is then passed to a condenser at the heat sink where heat is subsequently given off on condensation. In this manner it can be achieved that the heat transfer to the fluid at the evaporator takes place at a lower temperature than heat transfer away from the fluid at the condenser.
Although such systems provide effective cooling, considerable energy is required to compress the fluid. Refrigerators and domestic air conditioning systems are in widespread use worldwide and their ever increasing consumption is a cause for great concern. Efficiency can be improved by the use of better working fluids and more efficient compressors. The usual manner of comparing efficiency is in terms of the coefficient of performance COP. This is generally calculated as the ratio of the rate of energy removed from the heat source to the power input to the system (usually the electrical power supplied to the compressor). The COP is strongly dependent on the temperatures at the source and sink and is difficult to compare overall. It typically has a value of between 2.8 and 3.2.
Another form of cooling system that is used in certain situations is based on the principle of evaporative cooling. Unsaturated air having a relative humidity below 100% has a propensity to adsorb further water by evaporation. The latent heat required to evaporate the water causes cooling of the air, as its enthalpy remains substantially constant. This principle of direct evaporative cooling has been known for many centuries. For example, a damp cloth placed over an object will keep the object cool by evaporation of liquid from the cloth. By continuously adding liquid to the cloth, the cooling effect may be maintained indefinitely without input of electrical energy. For air that is cooled by direct evaporation its absolute humidity increases due to the uptake of moisture. Its relative humidity also increases due to its lowered temperature until it is full saturated with water vapour. The lowest temperature that can be reached by evaporation of moisture in this way into an air stream defines the wet-bulb temperature for that air. Direct evaporative coolers are also sometimes referred to as adiabatic coolers, since the air is cooled at constant enthalpy.
An indirect evaporative cooler exploits the same principle of evaporative cooling to achieve an improved cooling effect. A primary or product air stream passing over a primary surface of a heat exchange element may be cooled by a secondary or working air stream passing over and absorbing moisture from a secondary wetted surface of the heat exchanger. Because the air is cooled without direct evaporation, its absolute humidity remains the same. As its temperature decreases only the relative humidity increases. Cooling to below the wet bulb temperature may be achieved in this manner although the actual temperature reached will depend on the wet bulb temperature of the working air stream.
Ideally, cooling of the air to the so-called dew point is desirable. The dew point is lower than the wet bulb temperature and is defined as the temperature to which a body of air must be cooled to reach saturation or 100% relative humidity. At this point, water vapour in the air condenses. Attempts have been made to improve on the principle of indirect evaporative cooling to approach the dew point by cooling or drying the working air stream prior to evaporation taking place. A particularly convenient way of cooling the working air stream is to feedback a portion of the cooled product air. Such devices are often referred to as dew point coolers as they may lower the temperature of the product air to below its wet bulb temperature and close to the dew point. By optimising the surfaces with which the air streams exchange heat, highly effective heat transfer can be achieved. This has been found especially significant in the case of the heat transfer from the wetted secondary surface. In order to provide moisture to the working air stream, the wetted secondary surface may be provided with some form of liquid supply e.g. in the form of a hydrophilic layer. The presence of such a layer can however result in increased thermal isolation of the secondary surface from the working air stream, thus reducing heat transfer.
A particularly efficient form of dew point cooler is known from PCT publication WO03/091633, the contents of which are hereby incorporated by reference in their entirety. While not wishing to be bound by theory, it is believed that the success of this device is due at least in part to the presence of heat transfer elements on the primary and secondary surfaces. These heat transfer elements may be in the form of fins and are believed to improve transmission of heat from the primary surface to the secondary surface. The fins act both to directly conduct heat and also to break up the various boundary layers that develop in the flow. They also serve to increase the total area available for heat exchange on the relevant surfaces. Further important features of the wetted second surface are known from that document and also from PCT Publication WO2005/040693, the contents of which are also incorporated by reference in their entirety. Accordingly, by careful choice of the material used as a water retaining layer, optimal evaporation may be achieved without thermal isolation of the secondary surface from the working air stream.
Such devices are extremely convenient for cooling, as they are simple to produce and install and require no refrigerant or compressor. Air may be circulated through the cooler using a low-pressure fan which has low energy consumption and is relatively silent. This makes the dew point cooler ideal for domestic use, especially at night. They are also easily adaptable for heat recovery and ventilation. Such devices may also be defined by values for COP based on the cooling produced and the power input to the fan. Nevertheless, this actual performance is strongly dependent upon the atmospheric conditions and may vary from 60 for hot dry external air to 30 in cases of high humidity where the capacity for further evaporation is small.
Attempts have been made in the past to achieve all-round performance by combining the principles of evaporative cooling with conventional air conditioners. An air conditioning unit comprising an air to air heat exchanger in combination with a vapour-compression type cooling circuit is known from U.S. Pat. No. 7,093,452. A control system controls the device to operate with only the air to air heat exchanger when a room temperature is below a pre-determined level. If the temperature rises, both systems are operated together.
Another device is known from U.S. Pat. No. 5,970,723 in which a conventional air conditioner and a direct evaporative cooler are combined. The device is arranged to switch backwards and forwards between the two systems by movement of a damper element. Control is based on a number of possible inputs including external temperature and humidity and user defined values.
Although various such combinations have been proposed, none of the known devices is able to maximize the efficiency of operation under all conditions of temperature and humidity. Furthermore, the known systems have not been shown to adequately provide air at a predetermined and consistent supply condition, as is increasingly required by building designers.